1. Field of the Invention
The present invention generally involves chemical lasers. More particularly, the present invention involves an improved chemical laser configuration for space and ground applications.
2. Description of the Related Art
Conventional linear lasers provide a single chemical laser gain region from a combustion chamber as shown in FIG. 1. With this configuration, mass efficiency is limited by heat loss to the large surface area i.e., three sides of the combustion chamber. The high weight of the conventional laser is driven by the structural requirement to contain combustion gases at high pressure and high temperature. Finally, the medium quality of the conventional laser is degraded with increasing device length and power due to systematic optical path disturbances in gain of the medium that cannot be compensated.
The use of a chemical reaction to produce a continuous wave chemically-pumped lasing action is well known. The basic concept of such a chemical laser is described, for example, in U.S. Pat. No. 3,688,215, the subject matter of which is incorporated herein by reference. As therein described, the continuous wave chemical laser includes a plenum in which gases are heated by combustion or other means to produce a primary reactant gas containing dissociated atoms of a reactant element such as fluorine mixed with diluting gases, such as helium or nitrogen. The resulting reaction between the hydrogen (or deuterium) and fluorine produces vibrationally excited HF or DF molecules. These molecules are unstable at the low temperature and pressure condition in the cavity and return to a lower vibrational state by releasing photons. Mirrors spaced in the cavity along an axis transverse to the flow field amplify the lasing action from the released photons within the optical cavity formed by the mirrors. The lasing action is of the continuous wave type, which is pumped by the high-energy vibrationally excited molecules formed in the optical cavity. The lasing action depends on producing vibrationally excited states in the HF or DF molecules. This in turn requires that the molecules be formed under conditions of low temperature and pressure. As the pressure and temperature increase, the number of vibrationally excited molecules decreases and more energy goes into translational movement of the molecules, defeating the lasing action.
Cylindrical lasers as illustrated in FIG. 2 provide compact packaging of the gain generator, but require large volumes for handling the radial outflow of laser exhaust gas. End domes are required to contain the combustion products with atomic fluorine in the chamber. The domes are large surface area, heavy structural members that reduce mass efficiency from heat loss effects. Gain medium optical path disturbances increase with cylinder length and cannot be compensated, thereby limiting length and power scaling. Cylindrical combustion devices and optics for power extraction require stringent tolerances during fabrication and alignment, resulting in very high costs for a fragile beam generator. Conventional linear and cylindrical lasers experience large temperature gradients in the structure resulting in time-varying medium quality and laser performance. The radial flow of laser gas lowers the mass flux at the entrance to the diffuser, resulting in lower pressure recovery than linear flow devices.
A low-pressure hydrogen fluoride (HF) laser is a chemical laser, which combines heated atomic fluorine (produced in a combustion chamber similar to the one in a rocket engine) with hydrogen gas to produce excited hydrogen fluoride molecules. The light beam that results radiates on multiple lines between 2.7 μm and 2.9 μm. These wavelengths transmit poorly through the atmosphere. Conventional HF lasers utilize primary nozzles, referred to as hypersonic low temperature or HYLTE nozzles, the surfaces of which are smooth, curved planes that result in nearly parallel flow of gases at the exit of the nozzle. Helium and hydrogen cavity fuel are injected at oblique angles from the nozzle sidewalls. Mixing, reaction and laser gain are produced internal to the primary nozzles and in the downstream optical cavity region. A large base region is formed between adjacent primary nozzles. In a process referred to as helium base purge, helium or other gas must be introduced into these base regions to prevent recirculation of laser gas with ground-state HF that would reduce laser gain and mass efficiency. Conventional HYLTE nozzle configurations wherein hydrogen is injected with wall-jets, produces a gain internal to the primary nozzle and the large base region between the adjacent primary nozzles is subsonic helium flow that produces no gain. Further, there are flow regions at the laser cavity exit with unmixed atomic fluorine, hydrogen rich regions, and a large subsonic base flow region. These attributes of the conventional HYLTE nozzle result in inefficiencies within the HF laser and a significant loss of power.
There is a need in the art for a laser and nozzle configuration that reduces the inefficiencies currently found in the conventional configurations.